Does leaning whilst gliding reduce altitude?

Solution 1:

Leaning forward increases your glide ratio, meaning your forward speed increases, but you will still descend at the same rate as before. You can see this in game by looking at your character from the side while gliding. When leaning forward, the trail left by the glider is more flat compared to regular gliding:

The wiki page you linked mentions that under notes:

Leaning forward will reduce horizontal distance travelled for a given amount of endurance, but you will lose less vertical height than you would without leaning.

This means that when you want to reach a destination, such as a ledge across a ravine, leaning forward is better, because you lose less height overall. Of course, that is only if you can afford leaning forward in terms of gliding endurance (falling halfway is clearly not optimal).

In fact, there are several gliding "jumps" that are not possible without leaning forward all the way, for example the last glide to Herta Tablet #12 in Auric Basin (see 5th image).

In short:

• Regular gliding allows you to go farther per endurance lost.
  You can go X units regularly, and Y < X with lean gliding before your gliding endurance is empty.
• Leaning forward allows you to go farther per height lost.
  You can go X units regularly, and Y > X with lean gliding before you lost 10 meters of height.

Here's some screenshots of a small test I did. I jumped from the position seen in image 1. Image 2 shows where I landed without leaning, image 3 shows the landing point when leaning forward all the way. I passed that big fern by a couple of meters while leaning.

Solution 2:

You fall at a steady rate while gliding, regardless of how you lean. Leaning forward will increase your horizontal movement speed (at the cost of endurance) and leaning backward will decrease your horizontal movement speed, but you will continue descending at a steady rate.